The present invention relates to the field of transducers. More particularly, the present invention relates to an omni-directional ultrasonic transducer apparatus.
In the environment of transducers, it is known that an ultrasonic transducer may be formed with either a linear or curved film incorporated therein. In the event that a curved film is used, the ends of the curved film are butted up against each other with no apparent connection between the butted ends. Each of the types of film is described in the following.
Referring first to FIG. 7, a linear polymer piezoelectric film 70 is shown. When an AC voltage is applied to electrodes 72 on surfaces of the film 70, the film length in the molecular chain direction shrinks or expands. In other words, the PVDF film (the polymer piezoelectric material) is stretched during the process and molecular chains are aligned in parallel. This is due to excitation in the linear direction.
Alternatively, a cylindrical piezoelectric film 74 is shown in FIG. 8 whereby the stretched axis is wrapped around a cylinder (not shown). Here, when an AC voltage is applied to electrodes 76 on surfaces of the cylindrical film 74, the length vibration is converted to radial vibration. This is the principle of PVDF tweeter as disclosed in xe2x80x9cElectroacoustic Transducers with Piezoelectric High Polymer Filmsxe2x80x9d. J. Audio Eng. Soc. Vol. 23, No. 1, pp. 21-26, (1975) by M. Tamura et al. The high polymer element in the piezoelectric film is a poly-vinylidene fluoride) (PVDF) in film form.
The cylindrical PVDF vibrator has a certain mass and stiffness for radial expansion or shrinkage, and this mass and stiffness enable a resonance whose frequency is
f0=(1/2pR)xc3x96Y/rxe2x80x83xe2x80x83(1)
where
R is the radius in meters, Y is Young""s modulus (N/m2), and r is density (Kg/m3).
This equation is shown in a paper by A. S. Fiorillo entitled xe2x80x9cDesign and Characterization of a PVDF Ultrasonic Range Sensorxe2x80x9d. IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Controlxe2x80x9d. Vol. 39, No. 6, pp. 688-692 (1992), which is for semi-circularly curved film with both ends clamped, but it has the same resonance frequency as a cylinder.
In the IEEE paper, the cylindrical PVDF film is mounted on a conventional smooth-sided spool. The radius of the spool determines the resonance frequency through equation (1). The PVDF film can be directly wrapped around a cylindrical surface of the spool with almost no gap between the surface of the film and the surface of the spool. Even though the appearance is of no gap, the film is actually supported on the spool by many tiny points of surface roughness. It has been determined that most of the supported area has gaps of from 2-20 microns between the contacts of the many tiny points of surface roughness. Since actual vibration amplitudes are about 1 micron peak to peak for a 150 Vpp drive, there are enough spaces to vibrate and actually permit the device to work. In addition, the film is bonded to the underlying device or cylinder in order to secure the film and prevent its movement.
However, in the known application of a film to a spool as described, a problem has been discovered by the inventors of the instant application. Specifically, it has been discovered that with the xe2x80x9cgap-freexe2x80x9d wrapping of semi-cylindrical or cylindrical film attempted in the known art, there are problems of uncontrollable resonance frequency and a reduced vibration of the PVDF film. In addition, it has been discovered by the instant inventors that the bonding of the film to a film support, such as a cylinder, contributes to the problems of uncontrollable resonance frequency and reduced vibration of the PVDF film.
In other words, since the air found in the 2xcx9c20 micron region (the xe2x80x9cback air spacexe2x80x9d) has stiffness and spring effect, this also increases the effective stiffness of the PVDF film and in turn increases the resonance frequency of the film. Also, many points of contact are present between the cylinder and the PVDF film, including the bonding of the film to the cylinder, such that energy is lost due to friction, and the vibration of the PVDF film is thereby reduced. Since a thickness of the back air space is not controlled in the known art, nor recognized that it could or should be controlled, the resonance frequency also can not be controlled. Instead, it has been discovered by the inventors that if back air thickness exceed a certain value, the spring effect of the back air becomes less and even becomes negligible. In accordance with the principles PVDF film around a cylinder, when the PVDF film is wrapped, a seam may be created at the joined ends of the film. The inventors have discovered that certain connections at the seam location, instead of bonding the PVDF film to the cylinder, will significantly reduce the ultrasonic wave pressure radiated from that side of the cylindrical film, thereby improving the reliability of the device. In effect, the inventors have discovered that the seam should vibrate at the same frequency as the remainder of the film to achieve an optimum operation of the transducer apparatus.
Accordingly, a need in the art exists for an ultrasonic transducer apparatus in which a sealing or fixing of the vertical edges of the PVDF film will not significantly reduce the ultrasonic wave pressure radiated from that side of the cylindrical film. Enabling of the joined edges of the PVDF film to vibrate has been discovered by the inventors to enhance the effect of the transducer apparatus in a manner not heretofore known in the art.
It is an object of an embodiment of the invention to provide an ultrasonic transducer apparatus having a controlled resonance frequency.
It is another object of an embodiment of the invention to provide an ultrasonic transducer apparatus having a wrapped film surrounding a spool.
It is yet another object of an embodiment of the invention to provide an ultrasonic transducer apparatus wherein edges of the film surrounding the spool are joined so as to substantially enhance ultrasonic wave pressure radiating from that location.
It is a further object of an embodiment of the invention to provide an apparatus and method for attaching the piezoelectric film to a printed circuit board via ultrasonic staking.
It is a still further object of an embodiment of the invention to provide a cost effective ultrasonic transducer apparatus for eliminating the problems found in the known art of ultrasonic transducers.
These and other objects of the present invention are achieved by providing a transducer apparatus having a spool member with a body portion and first and second elevated regions formed on the body portion. A PVDF film surrounds the spool member, the film including an inner surface facing the spool member and an outer surface opposite the inner surface. The film as surrounding the spool member has a predetermined frequency of resonance. Lateral ends of the film are secured together by a securing material. The securing material is such that the secured ends of the film will have substantially the same resonance frequency as a remainder of the film. The film includes a non-electrode area at a perimeter of the inner surface and an electrode material formed on a remainder of the inner surface. Upon securing the lateral edges of the film together, the securement is at overlapping non-electrode lateral edges of the film. The securing material may be any one of an adhesive in combination with screws or thermally deformable nails, adhesive alone, tape, or ultrasonic welding.
Advantages of an embodiment of the invention as described more fully hereinbelow include a cost effective assembly for providing an ultrasonic transducer assembly having improved resonance and reduced vibration. This is accomplished by uniquely connecting joined edges of a PVDF film wrapped around a spool of the transducer assembly.
Additionally, the ultrasonic transducer of the instant disclosure has improved mass production ability and reduces the complexity and cost previously associated with the use of ultrasonic transducers in a manner not previously known in the art.